Phase 4 Stage 3: payment shell (multi-screen flow)

Six new screens under /payment/* + a paymentDraftProvider holding
mode/durationId/durationMinutes/priceIDR/paymentId/isFirstSessionDiscount
across the flow. PaymentEntryScreen handles the routing decision
(eligible+enabled -> /payment/discount-paywall, else /payment/method-pick)
and clears the draft on fresh entry.

Screens:
- discount_paywall_screen: S6 first-session discount with struck-through
  gimmick price + actual price + 'mulai · Rp{actual}' CTA -> /payment/method
- method_pick_screen: chat vs call cards
- duration_pick_screen: tier list with chat|call mode toggle that resets
  the selection on swap
- payment_method_screen: QRIS-first list, posts to existing
  /api/client/payment-sessions with mode/duration/price/discount/method
- waiting_payment_screen: qr_flutter QR (encodes paymentId in mock mode),
  20-min countdown header, 3s polling for status, pauses on background
  via WidgetsBindingObserver
- payment_expired_screen: retry CTA -> /payment/method with draft retained

Status mapping: real payment_sessions.status uses 'confirmed'/'consumed'
for paid (not 'paid' as in plan) and 'expired'/'abandoned' as terminal.

home_screen 'Mulai Curhat' CTA now pushes /payment/entry.

Dev-only /internal/_test/force-expire-payment endpoint to drive Maestro
flow 04_payment_expired.yaml without waiting 20 minutes. Gated behind
NODE_ENV !== 'production'.

chat_opening_provider PricingData extended to carry Phase 4 chat/call
groups + firstSessionDiscount, back-compat with the Phase 3 shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/lib/features/payment/screens/payment_entry_screen.dart

@@ -0,0 +1,67 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
+import '../../../core/chat/chat_opening_provider.dart';
+import '../../../core/theme/halo_tokens.dart';
+import '../state/payment_draft_provider.dart';
+
+/// Single point of truth for the discount-vs-method-pick branch.
+///
+/// Reads `chat-pricing.first_session_discount.eligible`. When the customer
+/// is eligible (and the discount is enabled), routes to the S6 paywall;
+/// otherwise routes to the regular method-pick screen. The draft is reset
+/// here so a fresh entry into the flow always starts clean.
+class PaymentEntryScreen extends ConsumerStatefulWidget {
+  const PaymentEntryScreen({super.key});
+
+  @override
+  ConsumerState<PaymentEntryScreen> createState() => _PaymentEntryScreenState();
+}
+
+class _PaymentEntryScreenState extends ConsumerState<PaymentEntryScreen> {
+  bool _routed = false;
+
+  @override
+  void initState() {
+    super.initState();
+    Future.microtask(() {
+      if (!mounted) return;
+      ref.read(paymentDraftNotifierProvider.notifier).reset();
+    });
+  }
+
+  void _routeOnce(String location) {
+    if (_routed || !mounted) return;
+    _routed = true;
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted) return;
+      context.go(location);
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final pricingAsync = ref.watch(chatPricingProvider);
+
+    return Scaffold(
+      backgroundColor: HaloTokens.bg,
+      body: pricingAsync.when(
+        loading: () => const Center(child: CircularProgressIndicator()),
+        error: (_, __) {
+          // Pricing fetch failed — fall through to method-pick (which fetches
+          // pricing again and surfaces the error there).
+          _routeOnce('/payment/method-pick');
+          return const SizedBox.shrink();
+        },
+        data: (pricing) {
+          if (pricing.firstSessionDiscount?.eligible ?? false) {
+            _routeOnce('/payment/discount-paywall');
+          } else {
+            _routeOnce('/payment/method-pick');
+          }
+          return const SizedBox.shrink();
+        },
+      ),
+    );
+  }
+}